In the field of color printing, it is well known that in order to obtain stable grayscale expression, portions coated with pigments are arranged in a square lattice array, and the tilt angles of the lattices of the respective color screens are made different from each other. Portions coated with pigments are called halftone dots. In color printing, if halftone dots are overlaid while the tilt angles of the respective color screens are the same, since the mutual positional relationship between the halftone dots is constant, screen misalignment directly causes color variations, resulting in unstable color appearance. For this reason, the tilt angles of the halftone dots are changed for the respective color screens, and the mutual positional relationship between the halftone dots between different color screens are made to differ from each other. As a consequence, the color screens slightly shift from each other, thereby canceling out variations in the overlay amount of the halftone dots due to the slight misalignment of the color screens (see, for example, Japanese Patent Laid-Open No. 6-130656).
There is a demerit in changing the tilt angles of halftone dots. When halftone dots are tilted, since the horizontal and vertical axis periods of the halftone dots differ from each other, a period until the mutual relationship between the halftone dots of the respective color screens returns to the same position is visually recognized as a characteristic pattern (moiréfringes).
In an electrophotographic printer, a potential image optically drawn on a photosensitive body is developed with a charged pigment, the developed image is transferred onto a paper sheet, and the image is fixed with heat and pressure, thereby performing printing. In contrast to this, in conventional printing, ink is applied to an image on a printing plate with a physically irregular surface or an image formed on a plane using a hydrophobic/hydrophilic pattern, and the plane is brought into tight contact with a paper sheet, thereby transferring the image onto the paper sheet. On printed matter, a solvent, containing a pigment, mostly volatilizes and leaves the pigment. In contrast, in the case of the electrophotographic printing scheme, an entire resin containing a pigment called toner remains as an adhesive for joining a paper sheet and the pigment. As a result, the volume of a substance remaining on the printed surface of a printout from a electrophotographic printer is larger than that in the conventional printing method. That is, a resin component remains to some thickness. In addition, when color printing is to be performed by the electrophotographic scheme, such resin components are overlaid.
A merit of a dry printing scheme is that there is no need to perform drying and processing for a solvent which has volatilized upon drying.
In the electrophotographic scheme of overlaying resin components having volumes, the following problem arises. The following problem arises because in the electrophotographic printing scheme, heat and pressure are applied to fix a pigment. A resin having a thickness is spread flat upon application of pressure thereon (spread phenomenon). The area of the resin spread becomes conspicuous as the thickness increases. That is, this phenomenon becomes more conspicuous at a portion where pigments overlap in color printing. Owing to this phenomenon, the grayscale expression scheme is influenced by the coating ratio of a pigment on a paper sheet area of printed matter where fine grayscale expression is made. More specifically, as a pigment is spread, the coating ratio of the pigment increases, resulting in an increase in print density.
It is not uncommon that an actually printed area increases or decreases as a whole relative to a drawn area. However, a color output from an electrophotographic printer has inconstant spread areas, and a portion where more toners overlap spreads more. In a portion where halftone dots of different colors overlap, in particular, the area of pigments increases, and the density increases. As a result, a change in density due to interference between colors becomes conspicuous. Such a change in density sometimes appears as an interference pattern (process moiré) stronger than a color screen interference in the original printed matter. This problem becomes more conspicuous in a printing system using, in addition to four colors (cyan (C), magenta (M), yellow (Y), and black (K)), printing materials which have the same hues and different densities to improve the color reproducibility.
For this reason, in the electrophotographic scheme, it is hard to realize grayscale expression by the same scheme as in the general printing scheme. In the general printing scheme, a pattern like that shown in FIG. 7A is used. In contrast to this, the electrophotographic scheme alternatively uses a line screen like that shown in FIG. 7B, halftone dots arranged in a parallelogram like that shown in FIG. 7C instead of a rectangular lattice, or a grayscale expression in a random dot pattern like that shown in FIG. 7D. Selecting a texture like that shown in FIG. 7B or 7C in grayscale expression makes it possible to increase the stability of print quality and avoid a halftone dot interference between color screens.
If, however, such a pattern is used, the smoothness of the boundary of edges greatly changes depending on the angle viewed. In a pattern like that shown in FIG. 7D, pixels are isolated, and hence this grayscale expression is not suitable for the electrophotographic scheme.
In addition, since a texture as a grayscale expression differs from printed matter, it is difficult to partly replace part of commercial printing. In order to inherit conventional printing techniques, it is important to reproduce such print textures. Incapability to express an image equivalent to commercial printed matter in addition to a demerit of an expression texture makes it impossible to meet demands for emulation such as proofing of printed matter. If output textures differ for different output devices, it is difficult to make the qualities of outputs by on-demanding printing uniform. It is preferable that printout textures be uniform regardless of output devices.